This proposal is for renewal of a current research grant which is concerned with several aspects of the natural forms of the vitamin, folic acid. We propose to carry out the following: 1. a rapid, functional assay for folic acid status has been developed using human lymphocytes. This is based on the incorporation of radioactive formic acid into a nonvolatile form. The details of this method have been worked out and values for a number of normal individuals have been obtained. We will apply this method to individuals who have folic acid deficiency as measured by the standard microbiological assay and determine whether there is a correlation between the two methods. 2. The folic acid binding proteins which we have identified in rat liver will be purified and characterized. The type and form of the folic acid bound by each of these proteins will be determined. Metabolic and storage roles of these binding proteins will be investigated by examining the effects of various deficiency conditions and drug on the extent of binding. Structural relationships between the folate binding proteins of liver and those which circulate in serum and milk will be investigated using immunological techniques. 3. A search will be made for folate binding proteins in tissues other than liver. 4. The importance of the kidney in the metabolism of folic acid will be examined. This will be studied in perfused kidneys and in nephrectomized animals.